Gas mainly composed of H.sub.2 and CO, obtained by partial oxidation of heavy oil or coal contains impurities, such as hydrogen sulfide (H.sub.2 S), COS and HCN. As one of methods of removing H.sub.2 S from the gas, a wet absorption method using methylenediethanolamine (MDEA) is known. However, since this method aims to remove H.sub.2 S selectively, the rate of removal of COS is low. Further, in this method, HCN reacts with MDEA to form a stable compound so that MDEA is deteriorated. Therefore, it is necessary to carry out a conversion treatment for COS and HCN as a pretreatment.
The pretreatment is a treatment for converting COS into H.sub.2 S which can be easily treated with MDEA, and HCN into NH.sub.3 which does not adversely affect an MDEA solution treatment, through conversion reactions represented by chemical formulae (1), (2) and (3) given below. EQU COS+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 S (1) EQU HCN+H.sub.2 O.fwdarw.NH.sub.3 +CO (2) EQU HCN+3H.sub.2.fwdarw.NH.sub.3 +CH.sub.4 (3)
Alumina catalysts are normally used for the conversion of COS. For example, catalysts in the state of alkalized alumina carrying potassium hydroxide or its salt reported in JP 5-70500 B2 and JP 768528 B2 and a catalyst made from alumina and barium oxide reported in JP 5-4133 B2 are used as active alumina catalysts.
It has been experimentally confirmed, however, that although each of the foregoing alkalized alumina catalysts is excellent in COS converting activity, alumina thereof changes into boehmite even in the presence of steam of only about 0.3 atm so that the COS converting activity is significantly deteriorated. Further, those catalysts have substantially no or less HCN conversion activity. On the other hand, the foregoing catalyst made from alumina and barium oxide is excellent in COS converting activity at low temperatures and is prevented from changing into boehmite under steam. However, it has also been confirmed that its activity with respect to HCN conversion is very low.
Under these circumstances, there has been a demand for a catalyst whose COS conversion activity is prevented from deterioration under steam and whose HCN converting activity is also high. As such a catalyst, there is an alkalized chromium oxide-aluminum oxide catalyst as reported in JP 5-80257 B2.
However, the processing speed of the alkalized chromium oxide-aluminum oxide catalyst is low. Thus, when converting a large amount of mixed gas containing COS and HCN, the processing takes a long time. For shortening the processing time, a large amount of the catalyst is required thereby to increase the size of an apparatus and thus the cost thereof.
From the industrial viewpoint, it is required that upon converting the foregoing mixed gas by contacting it with a catalyst in the presence of the steam, the mixed gas and the steam at a volume ratio of 0.05.ltoreq.steam/mixed gas.ltoreq.0.3 be contacted with the catalyst at a gas hourly space velocity no less than 2000 h.sup.-1 at a temperature in the range of 150.degree. C. through 250.degree. C. for a long time. Accordingly, it is required that the processing speed of the catalyst be high and the lowering of the catalyst activity be suppressed even during the long-time processing.
The grain size of the foregoing alkalized chromium oxidealuminum oxide catalyst is about 5 mm to 6 mm. In general, it is known that the activity of a catalyst increases as the geometrical surface area per unit weight of the catalyst increases. In this regard, the present inventors have aimed to obtain a catalyst suitable for mass-treating the mixed gas on an industrial scale by optimizing the size of the alkalized chromium oxide-aluminum oxide catalyst.
The present invention has been made under such circumstances and has an object to provide a conversion process for COS and/or HCN contained in a mixed gas, wherein upon implementing a conversion mass-treatment for COS and/or HCN contained in the mixed gas, COS and/or HCN can be converted at the high processing speed and further the high conversion rate of COS and/or HCN can be achieved even during the long-time processing.